Zombies attack: ants helpless in clutches of parasite

The hot summer day has cooled. The grass is tall. Impelled by strange forces it neither knows nor understands, the ant climbs to the top of a grass stalk, clamps its jaws around it, and waits.

Its insect brain, addled by ingestion of a parasite, is no longer in control of its actions.

Yes, it’s a zombie ant, and it is part of a bizarre life cycle involving snails, ants, cattle and liver flukes. It is a story so strange that it has to be real.

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Cam Goater, a biology professor at the University of Lethbridge who is researching the phenomenon, supports the zombie description.

“It’s of course accurate in that the ants are essentially under the control of another organism, in the sense that they’re forced to go up the vegetation and clamp on, and normally they wouldn’t do that. The way we view the zombie part of it is that they’re forced to do this every day for the rest of their lives.”

The story begins with the lancet liver fluke, which came to eastern North America from Europe in the 1950s and has moved westward as far as Cypress Hills Provincial Park.

Fluke eggs are eaten by snails. Larvae develop and the snail coughs them up as slime balls.

Along comes the ant. It’s a common ant, the red and black kind that people don’t like because they bite.

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The ant eats the slime ball as a moisture source. Larvae within the slime ball mature and commandeer a cluster of nerves that control the ant’s actions.

As summer temperatures cool each evening, the ant is compelled to climb to the top of tall grass and stay. Cattle, which typically feed in the cooler evening and morning, eat the ant along with the grass.

Once inside the animal, the flukes work their way to the bile duct and lay eggs. Some are shed in manure and the cycle begins anew.

“This parasite, like many of them, it’s very complex in terms of the life cycle,” said Goater. “You need all the partners in one small area for the thing to work. It’s amazing that it works at all, in fact.”

The liver fluke life cycle is significant to the cattle industry because flukes’ existence in cattle can cause livers to be condemned at slaughter. Heavy infestations can also make cattle anemic and prone to other health problems.

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Goater and livestock parasitologist Doug Colwell of Agriculture Canada have researched liver flukes for years, but in another twist to the zombie ant tale, it took a three-year-old boy to actually find one.

“Doug and I had been frustrated for a number of years because we knew that the cattle and the elk and all those animals are infected in the (Cypress Hills) park, in their livers. That’s the final host.

“So we knew that the only way they could get infected is to eat clinging ants, but for years, for two or three years, we looked and looked and looked in the Cypress Hills for these clinging ants and never found one,” said Goater.

Students joined the search. Nothing.

Then Goater took his family on a trip to the park.

“It was actually my son who found the first zombie ant. He was only three at the time. He’s sort of at the right height.”

Excited by the find, Goater immediately dissected the ant, “much to the delight of my wife,” and confirmed the infection.

“Now we know exactly where to go and find them,” he said.

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The ants have been discovered only in the provincial park, although the search has gone much farther. Goater speculates the unique combination of the park environment, grazing cattle, fire control and moisture conditions coincide to allow the life cycle.

Moist areas of the park near aspen stands are the ideal environments.

However, that isolation may not last. Goater said any animal that eats the ant will become infected. And yes, that includes humans who might casually chew on a blade of grass.

“Certainly our prediction from the work that we’re doing is that it’s likely to spread to other areas.”

That means management and conser vation implications for cattle producers and wildlife.

In the latter case, Goater and his colleagues continue to research the fluke’s effect on elk and deer.

“This parasite can get into wild animals. It can get into deer and elk and everything else and we certainly have no idea what it does to them.”